Tandy 1000 SL Technical Reference Manual page 193

pPD76SA/7265
NEC
data is read during the result phase, the interrupt is au-
tomatically cleared (INT
=
0).
The R-ormsignals should be asserted while E K Is
-
true. The =signal is used in conjunction with and
W R as a gating function during programmed 110 opera-
tions.
=
has no effect during =operations. If the
non-DMA mode is chosen, the DACK signal should be
It is important to note that during the result phase all
bytes shown In the command table (table 4) must be
read. The read data command, for example, has seven
bytes of data In the result phase. All seven bytes must
be read in order to successfully complete the Read Data
command.ThepPD765A/pPD7265wlll not accept anew
comand until all seven bytes have been read. Other
commands may require fewer bytes to be read during
the result phase.
The pPD765AIpPD7265 contains five status registers.
The main status register mentioned above may be read
by the processor at any time. The other four status regis-
ters (STO, ST1, ST2, and
ST3)
are available only during
the result phase and may be read only after completing
a command. The particular command that has been ex-
ecuted determines how many of the status registers will
be read.
The bytes of data which are sent to the pPD765AI
pPD7265 to form the command phase and are read out
of the pPD765AIpPD7285 in the result phase must occur
in the order shown in table 4. That is, the command code
must be sent first and the other bytes sent in the pre-
scribed sequence. No foreshortening of the command
or result phases is allowed. After the last byte of data in
the command phase is sent to the pPD765AIpPD7265,
the execution phase automatically starts. In a similar
fashion, when the last byte of data Is read out In the
result phase, the command is automatically ended and
the pPD765AIpPD7285 is ready for a new command.
pulled up to
vcc.
Polling
After reset has been sent to the pPD765AlpPD7285, the
unit select lines USoand U a will automatically go intoa
polling mode. In between commands (and between step
pulses in the Seek command) the pPD765AlpPD726.5
polls all four FDDs looking forachange In the ready line
from any of the drives. If the ready line changes state
(usually due to a door opening or closing), then the
pPD765AlpPD726.5 will generate an interrupt. When sta-
tus register 0 ( S O ) Is read (after Sense interrupt Status
is issued), not ready (NR) will be indicated. The polling
of the ready line by the pPD765AIpPD7265 occurs con-
tinuously between commands, thus notifying the proc-
essor which drives are on or off line. Each drive is polled
every 1.024ms except during the ReadlWrlte com-
mands. When used with a4 MHz clock for interfacing to
minifloppies, the polling rate is 2.048 ms. See figure 3.
Figure 3 . polling Feature
Read Data
A set of nine (9) byte words are required to place the FDC
into the read data mode. After the Read Data command
has been Issued the FDC loads the head (if it Is in the
unloaded state), waits the specified head settling time
(defined in the Specify command), and begins reading
ID address marks and ID fields. When the current sector
number(R) stored in the ID reglster(1DR) compares with
the sector number read off the diskette, then the FDC
outputs data (from the data field) byte-to-byte to the
main system via the data bus.
After completion of the read operation from the current
sector, the sector number I s lncremented by one, and
the data from the next sector Is read and output on the
data bus. This continuous read function is called a
multi-sector read operation. The Read Data command
may be terminated by the receipt of a terminal count sig-
nal. TC should be issued at the same time that the
DACK for the last byte of data is sent. Upon receipt of
this signal, the FDC stopsoutputtlng data to the proces-
sor, but will continue to read data from the current sec-
tor, check CRC (cyclic redundancy count) bytes, and
then at the end of the sector terminate the Read Data
command. The amount of data which can be handled
with a single command to the FDC depends upon MT
(multi-track), MF (MFMIFM), and N (number of bytes/
sector). Table 5 shows the transfer capacity.
The "multi-track" function (MT) allows the FDC to read
data from both sides of the diskette. For a particular cyl-
inder, data will be transferred starting at sector 1 , side 0
and completing at sector L, side 1 (sector L = last sector
on the side). Note, this function pertains to only one cyl-
inder (the same track) on each side of the diskette.
When N
=
0, then DTL defines the data length which the
FDC must treat as a sector. If DTL is smaller than the
actual data length in a sector, the data beyond DTL in
the sector Is not sent to the data bus. The FDC reads
(internally) the complete sector performing the CRC
check and, depending upon the manner of command
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